The present invention relates to a composite motion guiding unit and a composite motion guiding apparatus incorporating the same which are used in an arm of an industrial robot or the like for guiding motions in the axial and rotational directions.
Known motion guiding mechanisms include a ball screw and a ball spline. The ball screw has a shaft with ball screw grooves formed in the outer pheripheral surface thereof in a helical fashion and a nut fitted on the shaft with a large number of balls interposed therebetween so as to reduce any friction generated between the nut and the screw shaft and thereby allow a motion to be transmitted smoothly. The ball spline has a spline shaft with linear ball spline grooves formed in the outer peripheral surface thereof in such a manner that they extend in the axial direction, and an outer cylinder fitted on the spline shaft with a large number of balls interposed therebetween for guiding a linear motion smoothly and at the same time for transmitting torque.
The above-described known ball screw is only capable of changing a rotary motion to a linear motion, and the ball spline only has the function of guiding the linear motion smoothly while transmitting torque. Therefore, if these two functions are required in one apparatus, both the ball screw and the ball spline must be incorporated therein. This increases the number of parts and makes the structure of the device complicated.
A motion guiding mechansm incorporating a shaft body which has ball spline grooves and ball grooves is also known. In such a motion guiding mechanism, however, the ball spline grooves and the ball screw grooves are provided at separate portions of the shaft body, making the shaft body long and the size of the apparatus large. An increase in the length of the shaft body reduces the torsional rigidity, causing a strength-related problem. It also prevents any reduction in the size of the apparatus.